Routing and Wavelength Assignment (RWA) is a well-known problem for fixed grid optical networks while Routing and Spectrum Assignment (RSA) is its equivalent term to the same problem for flexible grid optical networks or gridless optical networks. In fixed grid optical networks, wavelengths are spaced apart from each other according to a wavelength spectrum grid defined by International Telecommunication Union (ITU) in ITU-T G.694.1 (02/12), “Spectral grids for WDM applications: DWDM frequency grid,” the contents of which are incorporated by reference. In flexible grid optical networks, which is also described in ITU Recommendation G.694.1 “Spectral grids for WDM applications: DWDM frequency grid” (02/12), each signal can be allocated to spectrum with different widths optimized for the bandwidth requirements of the particular bit rate and modulation scheme of the individual channels. Note, flexible grid networks may still utilize a grid, albeit at a much finer granularity than grid networks (e.g., 6.25 GHz vs. 50 GHz). On the other hand, gridless networks have no such grid constraints. The ultimate objective of RWA or RSA is to find a wavelength or spectrum assignment on a route for a particular channel in the optical network, such assignment and routing being optimal in some manner.
Existing approaches to RSA generally use a grid with either a sliding window or Integer Linear Programming (ILP)-based approaches. For RSA, in comparison with RWA, there is no concept of frequency spacing on gridless spectrum, i.e., each channel is allocated a spectral range instead of a number of fixed grid slots. Thus, it is difficult and non-optimal to adapt existing RWA techniques to support RSA on flex grid or gridless spectrum.